When the 137-foot sailing schooner approached the mouth of Boston Harbor — bow pointed toward the open sea — it was time to head back in again. So Katia DePina grasped the wooden wheel and pulled it deeply counterclockwise, and the 260-ton Roseway turned gracefully into the wind, changing course at the 13-year-old’s command.

Three days earlier, the Dorchester girl had stepped warily on board for the first time. Catching sight of the scuppers — narrow drainage slots along the edges of the deck — she worried that the ocean would flood in. “I was nervous,’’ the seventh-grader confessed, “because I’d never been on a sailing boat before.” Now, DePina was at the helm, steering the eye-catching vessel with the terracotta-colored sails in a wide arc toward the hazy, distant city.

Since mid-May, 380 Boston students have set sail on Roseway, where the nonprofit World Ocean School offers lessons in history, ecology, navigation, and, most essentially, teamwork. By the end of the summer — the schooner’s eighth as a floating classroom in Boston Harbor — 400 more will have made the boat their own. The school’s success in building skills and confidence comes from its unique setting aboard a historic 88-year-old sailboat, an environment where teamwork is required to make things work, says cofounder Abby Kidder.

“The mainsail weighs 2,000 pounds,” says Kidder, “so without all of these kids pulling together, it doesn’t go up. That’s not something we have to create, that’s the reality of sailing this boat . . . she is authentic, and the kids respond to that.”

BILL GREENE/GLOBE STAFF

Students raised the sails of the Roseway, an 88- year-old schooner. This summer, the crew will teach high-risk middle school students from Revere, and disabled students from Boston.

DePina was one of 36 seventh-graders from Dorchester’s Neighborhood House Charter School who spent four days on board in May. Many, like DePina, were fearful at first, because they had not been on boats or could not swim.

“The first day you ask who’s gone sailing and there are no hands,” said Sean Shirley-Davidson, a school administrator. “The last day, every hand goes up. . . . We want them to do something they would never normally do. Once they do, the likelier they are to do more new things.”

A tall ship built in 1925 in Essex, Roseway’s history includes long, successful stints as a fishing boat and a warship. The schooner guided Allied ships through Boston Harbor’s minefields during World War II, and spent 30 years guarding the harbor as a pilot ship before being retired. Its distinctive twin-masted silhouette is highly visible along the Seaport District waterfront, yet its revamped mission remains little known. Kidder and her crew are trying to change that, raising their profile and forging new relationships as demand continues to grow for their programs.

‘We want them to do something they would never normally do.’

Kidder, a Boston native, had spent years imagining a new kind of school at sea when she heard, in 2002, that Roseway would be sold at auction. The schooner had been transformed into a successful Maine-based charter fishing boat, but its latest owner had gone bankrupt.

Told by the bank that they could have the boat for $10 if they could take it away in 10 days, Kidder and her cofounder, Dwight Deckelmann, quit their jobs, consulted shipwrights, and towed the schooner to a Boothbay Harbor shipyard. There they undertook an 18-month, $1.3 million restoration project, doing much of the work on the boat themselves, with help from volunteers, hired shipwrights, and the shipyard’s owners.

Capturing imaginations with their vision, they found friends, family, and philanthropists to loan them money. Other supporters donated freezers, a generator, even a load of white pine trees hauled out of the woods by Clydesdale horses and cut into planks to replace the schooner’s aging deck.

From the start, they wanted to return Roseway to its longtime home in Boston Harbor. The boat, a National Historic Landmark, is one of only six surviving examples of the Grand Banks fishing schooners built from the 1820s through the 1930s.

The schooner spends winters in St. Croix, serving public schoolchildren there, and stops in Savannah, Ga., each fall and spring on its way to and from Boston, devoting November and April to students in that city. In Boston this summer, the 10-member crew of deckhand educators will teach high-risk middle school students from Revere, disabled students from a Boston elementary school, and youngsters from the South Boston Boys and Girls Clubs, among others. Some trips help the nonprofit pay the bills, like one for a group of Iowa college students who come to study the literature of the sea.

But the school’s income is not sufficient to cover all the demand from groups unable to pay. A collaboration with the Orchard Gardens public housing complex in Roxbury is one of several new partnerships in the works, but funding to cover it is uncertain, leaders said.

“We’re thinking about the next step to serve all the need, maybe [by adding] other ports or other boats,” said Kidder.

On the last day of their weeklong session on the schooner, the Dorchester students absorbed a lesson on salt marsh ecology, learning about gastropods and passing around empty crab shells. They studied the harbor’s history as deckhand Rebecca Gormley prodded them for dates — “What year did the Boston Tea Party happen, Jake?” — and directed their gaze toward Boston Light, the first lighthouse built in the United States.

Then Eden Leonard, the boat’s education director, descended. “You guys ready to navigate us back into the harbor?” she asked. The students gathered around a nautical chart and struggled to match the numbered red buoys sliding past in the water with the maze of markings on the map.

In a highlight of their week at sea, each student had a chance to climb the rigging, ascending 60 feet above the deck while classmates shouted encouragement. Gavin Doherty, a 13-year-old with an armful of colorful bracelets and blond ponytail, thought of opting out but reconsidered.

“I won’t get the chance to do this again,” she said.

The students were quick to point out that sailing a schooner is not a vacation. “You get to learn how to be a sailor,” said Savanna Hernandez, 13. “It’s fun, but it’s hard work.”

Still, she was certain it would not be her last sail.

“I’ll remember this for a long time,” she said, “and I would like to do it again.”